1. Field of the Invention:
The present invention relates to a method and a device for detecting the visibility of object image for adjusting focus of an optical system, being especially suitable for use in such optical instrument as a camera.
2. Description of the Prior Art
Conventionally, as a method and a device for detecting visibility of object image being suitable for adjusting focus of an optical instrument such as camera, etc., Japanese Patent Publication Nos. Sho 39-29120 and Sho 41-14500 filed by the present applicant, U.S. Pat. No. 3,562,785 filed by Mr. Dwin Richardson Craig (corresponding to West German Patent DAS 1173327 Specification and Japanese Patent Publication No. Sho 44-9501) are known.
The Japanese Patent Publication No. Sho 39-29120 is a method to detect visibility of an object image by utilizing the photo-electric effect of non-linear resistance - intensity of illumination characteristics appearing clearly in such photo-electric conversion element as cadmium sulfide or cadmium selenide, etc. and forming an image on the photo-electric conversion element by an objective lens. This is done for finding out the variation in the inner resistance and the variation in the electromotive force so as to detect the visibility of an object image.
Also, Japanese Patent Publication No. Sho 41-14500 constitutes an improvement of the device of the above mentioned Japanese Patent Publication No. Sho 39-29120. It proposes a device being so made as providing two photo-electric conversion elements having such photo-conductive substances mentioned above, such as CdS, etc., at positions relatively in front of each other with the proposed focal plane of the optical system being sandwitched therebetween. Both elements are connected connecting the both to a differential circuit to detect the differential output of the both elements caused by the focus adjusting state of the optical system, thereby focusing of the optical system is detected.
Further, U.S. Pat. No. 3,562,785 utilizes basically the principle of Japanese Patent Publication No. Sho 39-29120, and proposes that when an object image is projected by an optical system on an image receiving plane of two photo-electric conversion elements having non-linear variation characteristics of the intensity of illumination versus resistance value, one of the elements receives a clear image, then the other elements is to receive diffused image, so that the variation in the resistance value of the elements, generated as a result of the variation in light distribution on the image receiving plane of said elements being caused by the variation in the sharpness of the image, is detected. Thus the optical system is adjusted until the electric response signal by both elements reaches a maximum value, and the device adjusts the optical system using the electric response signal by both elements.
The above mentioned photo-electric effect in the non-linear resistance -- intensity of illumination characteristics of photo-electric conversion element means that the electric properties of said photo-electric conversion element, especially the resistance value, increases or decreases as the visibility of object image formed on the photo-electric conversion element increases. The fact that the amount of incident light per unit area of photo-conductive substance varies depending on the variation of the visibility of object image, and as the visibility becomes maximum there will be a remarkable deflection in the distribution of the amount of incident light into each point of photo-conductive substance.
In other words, it is based on the fact that the difference in the amount of light at the bright spot and of dark spot becomes greatest when the visibility is the maximum, thereby generating great difference in the resistance value of each part of the photo-conductive substance. However, the distribution of bright spot and dark spot of an object which normally exists and is photographed is very irregular and the difference in brightness of bright spot and of dark spot, that is, the contrast, is not necessarily high. Thus sufficient photo-electric effect can not be obtained only by forming the object image on the plane of photo-electric element having a certain extension using the above mentioned photo-conductive substance, therefore the detection of visibility of object image with high degree of accuracy is difficult.